Numerous synthetic and natural polymers are known to be available as matrices for controlled drug delivery systems (DDS) such as implants, microcapsules and micro- and/or nanospheres. Some of these polymers are non-biodegradable, for example polymethyl methacrylate(PMMA), polystyrene(PST), ethylene-vinyl acetate copolymer(EVA), polyethylene-maleic anhydride copolymers and polyamides. In the case of non-biodegradable polymers, a drug carrying polymeric implant or pellet utilizing any of these polymers has to be removed at the end of the release period. To avoid the necessity of removal and problems associated therewith it has been found desirable to develop controlled drug release polymeric devices based on biodegradable or bioerodible polymers. The use of biodegradable polymers avoids the removal of the device from the site of administration after the depletion of the drug. Biodegradable polymers can be designed to degrade in vivo in a controlled manner over a predetermined time period. Suitable biodegradable polymers for use in such sustained release formulations are well described elsewhere and include polyesters such as poly(d,l-lactide), poly(d,l-lactide-co-glycolide), poly (.epsilon.-caprolactone), poly (hydroxybutylic acid) and poly (aminoacids), poly (orthoesters), polyanhydrides and polyalkyl cyanoacrylates. These polymers gradually become degraded by enzymatic or non-enzymatic hydrolysis when placed in an aqueous, physiological environment. The main mechanism of in vivo degradation of polymers is hydrolytic degradation in which enzymes may also play a role. Important factors influencing hydrolytic degradation include water permeability, chemical structure, molecular weight, morphology, glass transition temperature, additives, and other environmental factors such as pH, ionic strength, site of implantation, etc.
Various microencapsulation techniques incorporating a drug into a biodegradable polymer matrix are taught in the art. Exemplary of these are: (a) phase separation by emulsification and subsequent organic solvent evaporation (including complex emulsion methods such as O/W emulsions, W/O emulsions and W/O/W emulsions); (b) coacervation-phase separation; (c) melt dispersion; (d) interfacial deposition; (e) in situ polymerization; (f) spray drying and spray congealing; (g) air suspension coating; and (h) pan coating. As exemplified in U.S. Pat. No. 4,652,441, a W/O/W (water/oil/water) double emulsion in-water drying process is a commonly used method for microencapsulation of water-soluble hydrophilic drugs such as peptides and proteins. However, this procedure presents various technical problems in preparation of microspheres and their use. For example, there is a requirement that a third component, such as gelatin, must be present in addition to the drug and the polymer utilized, e.g., a polylactic acid polymer. It is difficult to obtain microspheres in submicron order and there is a low rate of incorporation of the drug into capsules due to three-layer (W/O/W) structure. Moreover, there is an unstable release of the drug from the microspheres beginning with a burst effect induced by damage to, or rupture of, the thin polylactic acid wall of the microsphere.
U.S. Pat. No. 5,100,669 is drawn to polylactic acid type microspheres containing physiologically active substances and to a process for preparing the same. It is advantageous in that the active substance can be uniformly incorporated into the microspheres without loss of the activity and can gradually release the active substance for a long period of time of more than one week. In this patent there is taught the preparation of microspheres wherein the hydrophilic physiologically active substance and hydrophobic polylactic acid are uniformly mingled in a molecular order. By using a co-solvent such as acetonitrile-water mixtures or glacial acetic acid for the oligomeric polylactides and various drugs including peptides and proteins, the active substance can be uniformly incorporated into the microspheres without loss of activity and sustained release can be achieved without significant initial burst. However, in order to obtain the microspheres the use of an organic solvent is required.
U.S. Pat. No. 4,526,938 discloses the use of amphipathic, non-crosslinked, branched or graft block copolymers which have a minimum weight average molecular weight of 5,000 as carriers for the continuous release of polypeptide drugs. The hydrophobic block component, such as polylactide, is biodegradable and the hydrophilic block component, such as polyethylene glycol, may or may not be biodegradable. Such copolymeric compositions are capable of absorbing water to form a hydrogel when placed in an aqueous, physiological type environment. The use of an organic solvent is required which can be denaturing to polypeptides. U.S. Pat. No. 4,745,160 discloses a similar type of copolymer which has a minimum weight average molecular weight of 1,000 and is self-dispersible in water. Such copolymers are also useful for the sustained release of polypeptide drug formulations. The dosage forms in this patent are obtained by freeze drying a dispersion of a mixture of copolymer and peptide drug to obtain a powder. The powder is then subjected to heat and pressure to prepare a dosage form by compression moulding.
Hutchinson (WO 93/24150) teaches salts composed of a cation derived from a peptide containing at least one basic group and an anion derived from a carboxy-terminated polyester. The polyester is selected from those derived from hydroxy-acids or polycondensation products of diols and/or polyols with dicarboxylic acid and/or polycarboxylic acids. Typically, the polymer is a d,l-lactide/glycolide copolymer having one terminal carboxylic acid group per polymer chain. A process for the manufacture of such salts and their use as extended release pharmaceutical compositions are also disclosed. Typically, the peptide and carboxylic acid polymer are mixed in glacial acetic acid and lyophilized. The freeze dried product is then added to dichloromethane and solvent cast to obtain a film. Hutchinson then describes the use of the film in polymer melt-processing techniques, such as extrusion, and compression and injection molding, wherein elevated temperatures (preferably less than 100.degree. C.) are used to melt the polyester-drug salt in the preparation of an implant. Such solid dosage forms can be reduced to microparticulate forms by comminution or milling. Hutchinson contains an excellent discussion on polymeric hydrophobic/hydrophilic component and drug incompatibilities and other problems associated with the formation of microspheres using various solvents in dispersion or freeze drying techniques and is therefore incorporated herein by reference.
European Patent Application 0258780 A2 describes drug delivery systems (DDS) made up of ABA or AB block copolymers wherein one block is a poly(alkylene oxide) and the other blocks are glycolic acid ester/trimethylene carbonate. It describes the coextrusion of the polymers and a biologically active material at 60.degree.-115.degree. C. on a laboratory scale extruder. The ratio of active material is chosen to be 1-50% w/w but is preferably 25-50% w/w. The 1.5 mm diameter fibers can be cut into lengths or cryogenically ground through a 20 mesh screen to give particles which are capable of being injected, or the fiber can be directly implanted.
U.S. Pat. No. 4,438,253 describes multiblock copolymers obtained by transesterification of polyglycolic acid and hydroxyl-ended poly(alkylene glycol) such as polyoxyethylene and subsequent addition of an aromatic orthocarbonate such as tetra-p-tolyl orthocarbonate to further increase the degree of polymerization. Those materials were used for manufacturing surgical articles and a hydrolyzable monofilament fiber.
The release of a polypeptide from a polylactide polymer is often preceded by a significant induction period, during which no polypeptide is released, or is polyphasic which comprises an initial burst release from the surface of the device, a second period during which little or no polypeptide is released, and a third period during which most of the remainder of the polypeptide is released.
Various attempts have been made to solve or at least minimize this problem and improve the release profile. One method is to copolymerize lactic acid with glycolic acid to form poly(lactide-glycolide) copolymers. Another is to mix a peptide encapsulated in polylactide polymer with the same peptide encapsulated in other polymers or copolymers. Both of these methods are difficult to control during manufacture and administration and have not been totally successful in achieving the desired peptide release rate.
One attempt to solve the release rate problem is presented in U.S. Pat. No. 5,330,768. This patent discloses degradable polymeric matrices prepared by the physical blending of biodegradable hydrophobic polymers, such as polylactides, with nonionic hydrophilic copolymers, such as surfactant block copolymers of polyethyleneoxide (PEO) and polypropyleneoxide (PPO). Protein or peptide drugs are incorporated into the polymeric blends by mechanical mixing, or by solvent or melt casting. In aqueous solutions, these polymeric blends allegedly form gel like structures within a polymeric skeleton which provide for extended protein release and minimized initial protein burst as compared to the pure polylactide polymers. However, when polymer blends are prepared as microspheres, a modified solvent evaporation technique using double emulsion is employed which requires the use of solvents such as methylene chloride which must be evaporated into the surrounding atmosphere.
In vitro release of bovine serum albumin (BSA) from ABA triblock copolymers consisting of copoly(l-lactic acid-b-oxyethylene-b-l-lacticacid) (LPLA-PEO-LPLA) and copoly(l-lactic-co-glycolic acid-b-oxyethylene-b-l-lactic-co-glycolic acid)(LPLG-PEO-LPLG) as microspheres was studied by Youxin et al., J. Controlled Release 32 (1994) 121-128. However, the microspheres were prepared by a triple emulsion technique which employs an organic solvent such as methylene chloride. Continuous release of protein can be obtained by adjusting the composition of such ABA triblock copolymers. The introduction of hydrophilic PEO blocks into hydrophobic polyesters should promote effective water uptake and should also increase the permeability of parenteral delivery systems especially for water soluble drugs such as peptides and proteins. Both molecular weight decay and mass loss are accelerated in such ABA block copolymers by the rapid penetration of water into the microphase-separated system. Depending on PEO content and polyester ratio degradation rates can be adjusted. The release of polypeptides from the matrix of ABA block copolymers is controlled by the diffusion of the drug in the swollen matrix as well as by the degradation of the matrix.
Implantable polymeric drug delivery systems have been known for some time. A unique aspect of ABA type copolymer formulations, such as those disclosed by R. L. Dunn et al., U.S. Pat. No. 4,938,763 and 5,278,202, is the fact that when formulated, they maintain a liquid consistency which allows injection using 22/23 gauge needle. Once in contact with aqueous fluid the copolymeric network absorbs the fluid and sets into a gel matrix. The overall triblock copolymer composition can be manipulated to have a degradation time in the range of days, weeks or months. Depending upon the polymeric blocks used, i.e. polymer type, molecular weight, relative proportions, etc, the degradation rate can be adjusted by choosing the appropriate polymeric block components. These block copolymers are generally non-toxic and are well tolerated by the body and the system is easy to formulate. Drug laden copolymers of lactic and/or glycolic acids and ethylene oxide provide a fresh approach for a biodegradable implant since they can be easily injected, avoiding the use of surgical procedures. A gel matrix, similar to an implant, is formed immediately after injection on contact with an aqueous environment of extracellular fluid and the release of the drug takes place slowly through this formed matrix. The in vivo degradation rate of the copolymers, such as those of lactic and/or glycolic acids and polyethyleneoxide, can be controlled by using different mole ratios of the constituent monomers and molecular weight of copolymers. Their biocompatibility and biodegradability is also well established. The gel matrix once formed will release the drug in a controlled manner and then degrade to products which are easily metabolized and excreted. This approach incorporates the advantages of a drug delivery device implant while circumventing the need for surgery to place the implant prior to administration or remove it after the release is complete. However, the major drawback in the preparation of these copolymers is the use of undesirable and sometimes toxic organic solvents such as N-methyl-2-pyrrolidone, dimethyl sulfoxide, ethyl lactate, triacetin and triethyl citrate. These solvents are used because they can dissolve the polymer components and are also miscible with water.
Primarily due to the development of DNA-recombinant techniques, peptide and protein drugs are becoming more and more available in a large scale. However, because of the relatively short biological half-life of protein drugs and their rapid degradation by proteolytic enzymes in the gastrointestinal tract, repeated daily injections are generally needed to optimize drug effectiveness. Among many new DDS devices, injectable drug containing polymeric microsphere systems can provide a means for the safe and controlled parenteral administration of peptides and proteins. However, the formulation and delivery of these relatively high molecular weight peptide and protein drugs can present certain problems due to their relatively fragile nature when compared to traditional, smaller molecular weight drugs. In order to successfully employ polypeptides as pharmaceuticals, it is essential to understand the stability issues relevant to their formulation and delivery. Polypeptides undergo a variety of intra and intermolecular chemical reactions which can lead to their decline or loss of effectiveness as pharmaceuticals. These include oxidation, deamidation, .beta.-elimination, disulfide scrambling, hydrolysis, isopeptide bond formation, and aggregation. In addition to chemical stability, polypeptides must also retain their three dimensional structure in order to be effective therapeutic agents. Loss of this native conformation leads not only to loss of biological activity but also to increased susceptibility to further deleterious processes such as covalent or noncovalent aggregation. Furthermore, the large size of protein aggregates leads to other problems relating to parenteral delivery, such as decreased solubility and increased immunogenicity. H. R. Costantino et al., J. Pharm. Sci., 83, (1994) 1662-1669 "Solid-phase aggregation of proteins under pharmaceutically relevant conditions."
By knowing the various molecular pathways which contribute to aggregation of solid proteins, rational approaches for stabilization can be developed. The first approach is to specifically target the mechanisms involved. A second approach is to maintain the level of moisture activity within the protein at optimal levels. This may be achieved by storing the protein at optimal hydration levels or, in the case of sustained release devices, by choosing a microenvironment that will ensure lower water activities. The pH of the microenvironment can also be controlled. A third approach for stabilizing solid protein formulations is to increase the physical stability of lyophilized protein. This will inhibit aggregation via hydrophobic interactions as well as via covalent pathways which may increase as proteins unfold.
The providing of a functional biodegradable hydrogel microsphere system is very desirable from a protein stability point of view. As noted above, the critical role of water in protein structure, function, and stability is well known. Typically, proteins are relatively stable in the solid state with bulk water removed. However, solid therapeutic protein formulations may become hydrated upon storage at elevated humidities or during delivery from a sustained release device. The stability of protein drops with increasing hydration. Water can also play a significant role in solid protein aggregation for various reasons, i.e., (a) it increases protein flexibility resulting in enhanced accessibility of reactive groups; (b) it is the mobile phase for reactants and (c) it itself is a reactant in several deleterious processes such as .beta.-elimination or hydrolysis. Proteins containing between 6% to 28% water are the most unstable. Below this level, the mobility of bound water and protein internal motions are low. Above this level, water mobility and protein motions approach those of full hydration. Up to a point, increased susceptibility toward solid-phase aggregation with increasing hydration has been observed in several systems. However, at higher water content, less aggregation is observed because of the dilution effect. Also, dilution of proteins with polymers of various functionalities, such as polyethylene glycol, dextran, diethylaminoethyl dextran, and carboxymethyl cellulose, significantly increase the stability of the protein and reduces the solid-phase aggregation.
One general way to stabilize proteins against solid-state aggregation is to control the water content in the solid formulation and maintain the water activity in the solid protein at optimal levels. This level depends on the nature of the protein, but in general, proteins maintained below their "monolayer" water coverage will exhibit superior solid-state stability. However, when solid proteins are suspended within a polymeric matrix intended for sustained release, the control of water activity is not always straightforward. According to current FDA requirements, an acceptable protein drug containing pharmaceutical product should exhibit less than 10% deterioration after 2 years.--Cleland, J. L. and Langer, R. In formulation and delivery of proteins and peptides, ACS books, 1994.
However, as is clear from the above description, the known means for providing microencapsulated protein containing DDS devices present some positive features, but are disadvantageous in that the fabrication procedures are complex and organic solvents are usually required during the microencapsulation process. Moreover these procedures may adversely affect the stability of peptide and protein drugs. For example, when fabricating microspheres using a polylactic acid copolymer, such as shown in U.S. Pat. No. 4,745,160, a mixture of dioxane and water is used. A dispersion is formed because the polylactic acid and/or the polypeptide are not completely dissolved in the mixture. Instead of dioxane and water, glacial acetic acid can also be used as a solvent. In either event the dispersion is lyophilized to obtain a powder. The powder can then be subjected to heat and pressure for preparation as a film, sheet, cylinder, or pulverized product thereof.
From the above it is evident that the prior art does not teach formulations of biodegradable polymeric microspheres without using an organic solvent and/or compression moulding techniques.
As previously noted, many microencapsulation processes require the use of organic solvents. There are some concerns about the toxicity of residual solvents, especially when chlorinated solvents (e.g., methylene chloride, chloroform) are used during the microencapsulation process. Structural and pharmacological denaturation as well as the loss of biological activity are commonly observed when large molecular weight polypeptides are in contact with organic solvents.